Communication networks presently retained in the avionic field are based on the ARINC 664 standard.
These communication systems rely on intermediate communication equipment (also called intermediate system or IS) of the switch type as well as on end interfaces (also called end system or ES) localized in each of the pieces of equipment subscribed to the network.
Presently it appears that an application data is transmitted through a same piece of equipment on two distinct sub-networks A and B, intended for one or several pieces of receiving equipment.
Thus the application datum is encapsulated and then duplicated by the transmitter so as to be transmitted over both sub-networks.
A sequence number (or SN) is inserted into the frames.
One of the goals of this SN is to identify both frames, one of the network A and the other of the network B, derived from the same occurrence.
Both sub-networks A and B do not have any piece of physical equipment in common or any common physical link.
Thus both flows of data take physically segregated paths.
Each logical flow is identified by a virtual link in which the physical frames pass in transit.
At the piece of receiving equipment, the ARINC 664 standard allows various configurations of the ES:                Configuration 1: only one of two frames bearing the same SN is provided to the application interface. The frame transmitted up to the application interface is the entire frame which arrives first.        Configuration 2: both frames are provided to the application interface.        
The presence of redundancy A/B inter alia gives the possibility of guaranteeing the integrity of delay, i.e. the frame provided to the application interface is not too old at acceptable levels, these levels being set by the system requirements.
Indeed, according to the criticality of the application data, the certification authorities require observance of different security constraints.
Thus for application data relative to critical airplane functions, the security constraints are the most restrictive and should notably observe a qualitative principle, no simple failure should lead to a catastrophic event for the airplane.
Physical segregation of the paths A and B ensures that a simple failure at the network cannot have the consequence of the transmission of a too old frame to the piece of receiving equipment.
Indeed, if the simple failure occurs on the path of the network A, the frame from the network B will not have been subject to this failure.
In order that the delay integrity be not guaranteed, a second failure would then have to take place on the path of the network B.